The present invention relates to a device for Electron Paramagnetic Resonance Imaging using a high amplitude modulator. More particularly, the present invention relates to a device for one-, two- or three-dimensional Electron Paramagnetic Resonance (EPR) Imaging by employing a technique of radiofrequency or microwave spectroscopy that detects and measures the spatial distribution of free radicals, certain transition metal complexes, certain rare earth metal complexes, triplet state molecules and the like, by virtue of the presence of unpaired electrons in such species. It has potential application in leather industry for detecting the presence and distribution of chromium in leather. It may also be used in the drugs, pharmaceuticals and cosmetics industry. The device is also envisaged to have use for detecting the presence and distribution of radicals including the nitroxide radical in metabolites for medical purpose. Further, it has potential use in mineralogical applications and also in foodstuffs industry.
Electron Paramagnetic Resonance, which is also known as Electron Spin Resonance (ESR), is one of the spectroscopic tools to study the molecular structure of organic free radicals and inorganic complexes. The measurement is performed by locating the sample of interest (usually liquid or solid) in a suitable resonator (e.g. cavity, slow wave structure, etc.) placed in a spatially homogeneous magnetic field and irradiating it with electromagnetic (em) radiation whose frequency matches the characteristic precession frequency of the electron spins in the external field. The resonance frequency is given by the following equation       ω    0    =                    g        ⁢                  xe2x80x83                ⁢        β            ℏ        ⁢          B      0      
where g is the spectroscopic splitting factor (Landxc3xa9 factor, or simply the g-factor), xcex2 is the Bohr magneton, B0 is the intensity of the magnetic field and  is Planck""s constant divided by 2xcfx80.
This xe2x80x98first orderxe2x80x99 equation for the resonance frequency is usually modified by additional factors involving internal (or local) fields from other unpaired electrons and/or nuclear spins. The resonant absorption or emission of em radiation under these conditions is customarily recorded by varying the intensity of the external magnetic field across the resonance condition, holding the frequency of the em radiation constant in continuous wave (cw) mode. In order to minimize direct current (dc) drifts during the course of the field scan and to improve signal-to-noise ratio, it is customary to employ field modulation, typically at 100 Hz to 100 kHz, employing a set of modulation coils mounted in the resonator/cavity and perform phase sensitive detection (PSD) of the output of the em detector (e.g. Diode detector). Normally, EPR measurements are employed to access detailed information about the electronic structure, shape and dynamics of molecular species, the sample and magnetic field being made as spatially homogeneous as possible.
This prompted researchers to study molecular distributions as well as molecular structure in inhomogeneous systems by employing the imaging technique, where the information is accessed by locating the object of interest in a suitable resonator in a magnetic field that has spatial variation, i.e. gradients. Under these conditions, the resonance frequency is given by the following equation       ω    0    =                    g        ⁢                  xe2x80x83                ⁢        β            ℏ        ⁢          (                        B          0                +                  G          ·          r                    )      
where G indicates gradient vector, while r indicates the position vector in the sample.
The resolution of an EPR image is dependent on a number of parameters, including the intensity of the magnetic field gradient, the intrinsic width of the basic EPR resonance of the species in question (the xe2x80x98linewidthxe2x80x99), the signal-to-noise ratio per volume element of the sample, molecular diffusion processes, etc. In general, the EPR linewidth is of the order of several Gauss (1 Gauss xe2x96xa110xe2x88x924 Tesla), or several Megahertz in frequency units. The resolution R expected on the basis of the first two parameters may be given as:   R  =            Δ      ⁢              xe2x80x83            ⁢              v                  1          /          2                      G  
Here, xcex94xcexdxc2xd denotes the linewidth, while G denotes the gradient amplitude. As reported by Swartz et al (Journal of Magnetic Resonance, 84, 247, 1989), Eaton et al (xe2x80x9cEPR Imaging and in vivo EPR,xe2x80x9d CRC Press, Boston, 1991), Eaton et al (Concepts in Magnetic Resonance, 7, 49, 1994), Eaton et al (Chemical Physics Letters, 142, 567, 1987) and Symons et al. (Journal of Magnetic Resonance, 92, 480, 1991), EPR imaging has conventionally been carried out by continuous wave (CW) method using a standard EPR spectrometer, where the frequency of radiation is held constant, while the magnetic field is swept. Depending on the nature of experiment, gradient currents are turned on to do 1-Dimensional, 2-Dimensional or 3-Dimensional EPR imaging or spectral-spatial work.
It is customary to generate the gradient(s) by additional sets of anti-Helmholtz or Anderson coils located in the main magnetic field. When currents are passed through such water- or forced air-cooled coils, sizeable gradients may be generated, with amplitude upto about 1 Tesla mxe2x88x921 (100 Gauss cmxe2x88x921). One set of coils is typically employed for each of the. three orthogonal directions in space. Because the inverse of the EPR linewidth is typically short compared to gradient switching times, it is customary to acquire the signal in the presence of the gradient, then reorient the sample with respect to the gradient (e.g. by rotating the gradient, adjusting the current amplitudes in two sets of gradient coils) to obtain a two-dimensional image. Projection reconstruction of the resulting series of profiles, including suitable shift, deconvolution and back projection operations then yields the desired image.
The main objective of the present invention is to provide a device for Electron Paramagnetic Resonance Imaging, which obviates the special requirements and limitations stated above.
Another objective of the present invention is to generate the desired EPR images, without employing additional sets of gradient coils.
Yet another objective of the present invention is to operate the field modulation at a high amplitude typically more than 20 Gauss, in the range of 14-78 Gauss in the present system.
Still another objective of the present invention is to exploit the intrinsic inhomogeneity or gradient in the modulation field to generate the desired information.
One more objective of the present invention is to provide a device for imaging using high amplitude modulator with its intrinsic gradient, on existing cw EPR spectrometer/imaging systems.
Yet one another objective of the present invention is to provide an option to rotate the sample in the resonator.
The novelty and non-obviousness of the present invention lies in using a high amplitude modulator (16) enabling large field modulation amplitudes, typically at least 20 Gauss, to exploit the inherent gradient of the field modulation, thereby avoiding use of additional sets of gradient coils to generate two or three dimensional EPR images, whereby the additional expenditure for providing gradient coils, gradient amplifiers and the associated cooling system, as is essential in case of conventional EPR imaging systems, can be avoided.